Solvent annealing effect in polymer solar cells based on poly(3-hexylthiophene) and methanofullerenes

The self organization of the polymer in solar cells based on regioregular poly(3-hexylthiophene) (RR-P3HT):[6,6]-phenyl C-61-butyric acid methyl ester (PCBM) is studied systematically as a function of the spin-coating time t(s) (varied from 20-80 s), which controls the solvent annealing time t(a), the time taken by the solvent to dry after the spin-coating process. These blend films are characterized by photoluminescence spectroscopy, UV-vis absorption spectroscopy, atomic force microscopy, and grazing incidence X-ray diffraction (GIXRD) measurements. The results indicate that the pi-conjugated structure of RR-P3HT in the films is optimally developed when t(a) is greater than 1 min (t(s) similar to 50 s). For t(s) < 50 s, both the short-circuit current (J(SC)) and the power conversion efficiency (PCE) of the corresponding polymer solar cells show a plateau region, whereas for 50 < t(s) < 55 s, the J(SC) and PCE values are significantly decreased, suggesting that there is a major change in the ordering of the polymer in this time window. The PCE decreases from 3.6% for a film with a highly ordered pi-conjugated structure of RR-P3HT to 1.2% for a less-ordered film. GIXRD results confirm the change in the ordering of the polymer. In particular, the incident photon-to-electron conversion efficiency spectrum of the less-ordered solar cell shows a clear loss in both the overall magnitude and the long-wavelength response. The solvent annealing effect is also studied for devices with different concentrations of PCBM (PCBM concentrations ranging from 25 to 67 wt%). Under solvent annealing conditions, the polymer is seen to be ordered even at 67 wt% PCBM loading. The open-circuit voltage (V-OC) is also affected by the ordering of the polymer and the PCBM loading in the active layer.